IE 7 will only support well formed feeds:

Here in Windows, we’re working hard on Windows Vista Beta 2, and we've recently been doing some work on how we parse feeds.

Our years of experience in with HTML in Internet Explorer have taught us the long-term pain that results from being too liberal with what you accept from others. Hence, we’ve adopted the following overriding principle for IE 7 and RSS platform in Windows Vista:

We will only support feeds that are well-formed XML.

It will be interesting to see whether they can hold to that or not. There's a lot of content being put up by a lot of people, and a lot of it has small errors. The feeds here periodically have issues, for instance - often times from comments that get added to a post. Then there are the search feeds, which scrape from a variety of feeds, and often have encoding problems.

I'm not saying it's the wrong approach - it might help. The trouble is, when IE won't accept a feed, what's the liklihood that the end user will contact the author of the feed in question? Low, I'd say. It will be interesting to see how this plays out. If you check the very end of the post, it's clear that they left themselves an out, in case this doesn't work they way they would like it to:

That said, we do recognize that there is a great deal of variance in the actual content of RSS feeds, so we’ll be more liberal when it comes to what elements are required in a feed. We will post on exactly how we're handling different feeds in a future post.